The present invention is concerned with a negative pressure type booster arrangement which is used for the operation of a brake master cylinder of a vehicle and constructed from a working mechanism, a control valve mechanism, a servo mechanism and a liquid pressure generator mechanism as an integral unit.
In the prior art negative pressure type booster arrangement of this kind, the connection of a brake pedal with an input rod is effected with clevises. A force is applied on the brake pedal to operate the control valve mechanism, so that a difference in air pressure is brought about between a constantly negative pressure chamber and a variable pressure chamber by a booster piston of the servo mechanism. That difference in air pressure then gives rise to a booster output which, in turn, supplies a higher liquid pressure from the liquid pressure generator mechansim to a brake device. Thus, the conventional booster arrangement has the advantages that a lesser force impressed on the brake pedal is converted to a larger force for braking.
The prior art booster arrangement has the following disadvantages, however. As the arrangement includes the liquid pressure generator mechanism attached bodily to the servo mechanism etc., some limitations are placed on the position at which the booster arrangement is mounted in place. The prior art arrangement cannot be used with two-wheeled vehicles such as motorcycles which are operated by a brake lever or the like in a remote control manner, because of the fact that the brake pedal is connected directly to the input rod by means of clevises. A risk is also incurred in using the arrangement, since a sufficient braking force is not obtained due to increases in the lost strokes of the brake pedal. Besides, it is troublesome to adjust the braking force. Furthermore, there is a possibility that, when a push is given to the brake pedal to cause movement of the input rod and hence actuation of the control valve mechanism, the input rod may swing and clog an associated air inlet passage, rendering the servo mechanism inoperative.
On the other hand, the prior art booster arrangement includes a dust boot disposed between a cylinder member extending rearwardly from the booster shell and the input rod for the purpose of preventing foreign matter such as sand and dust from entering the control valve mechanism of said extension member. However, such a dust boot should permit an amount of air to be introduced into the working chamber via the control valve mechanism or into said extension member so as to compensate a change in the volume thereof, which may take place during the forward or backward movement of the input rod. To this end, various attempts have been or are being made; however, none of them offer any satisfactory results. There is still a strong demand for a simplified arrangement for the prevention of entrance of sand, dust, water and the like. The present invention provides a solution to these problems.